chapter 10
TRANSCRIPT
The Nervous System
Chapter 10
10.1: Introduction
Cell types in neural tissue: Neurons Neuroglial cells - also known
as neuroglia, glia, and glial
Divisions of the Nervous System
1. Central Nervous System (CNS) Brain Spinal cord
2. Peripheral Nervous System (PNS) Cranial nerves Spinal nerves
Divisions of Peripheral Nervous System
• Motor Division• Carries information to muscles and glands
• Divisions of the Motor Division:• Somatic – carries information to skeletal muscle• Autonomic – carries information to smooth muscle, cardiac muscle, and glands
• Sensory Division• Picks up sensory information and delivers it to the CNS
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Divisions Nervous System
Sensory division Sensory receptors
Motor division
Skeletal muscle
Brain
(a) (b)
Spinalcord Spinal
nerves
Cranialnerves
Central Nervous System(brain and spinal cord)
Peripheral Nervous System(cranial and spinal nerves)
Smooth muscleCardiac muscleGlands
AutonomicNervousSystem
SomaticNervousSystem
10.1 Clinical Application
Migraine
10.2: General Functions of the Nervous System
The three general functions of the nervous system:
Receiving stimuli = sensory function Deciding about stimuli = integrative function Reacting to stimuli = motor function
Functions of Nervous System
Motor Function Decisions are acted upon Impulses are carried to effectors
Sensory Function Sensory receptors gather information Information is carried to the CNS
Integrative Function Sensory information used to create:
Sensations Memory Thoughts Decisions
10.3: Description of Cells of the Nervous System
Neurons vary in size and shape
They may differ in length and size of their axons and dendrites
Neurons share certain features: Dendrites A cell body An axon
Neuron Structure
Myelination of Axons
White Matter Contains
myelinated axons Considered fiber
tracts Gray Matter
Contains unmyelinated structures
Cell bodies, dendrites
10.2 Clinical Application
Multiple Sclerosis
10.4: Classification of Neurons and Neuroglia
Neurons vary in function They can be sensory, motor, or integrative neurons
Neurons vary in size and shape, and in the number of axons and dendrites that they may have
Due to structural differences, neurons can be classified into three (3) major groups:
1. Bipolar neurons
2. Unipolar neurons
3. Multipolar neurons
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Classification of Neurons: Structural Differences
• Bipolar neurons• Two processes• Eyes, ears, nose
• Unipolar neurons• One process• Ganglia of PNS• Sensory
• Multipolar neurons• 99% of neurons• Many processes• Most neurons of CNS
Dendrites
Axon Axon
AxonDirectionof impulse
(a) Multipolar
Centralprocess
Peripheralprocess
(c) Unipolar(b) Bipolar
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Classification of Neurons: Functional Differences
• Sensory Neurons• Afferent• Carry impulse to CNS• Most are unipolar• Some are bipolar
• Interneurons• Link neurons• Aka association neurons or internuncial neurons• Multipolar• Located in CNS• Motor Neurons
• Multipolar• Carry impulses away from CNS• Carry impulses to effectors
Central nervous system Peripheral nervous system
Cell body
Interneurons
Dendrites
Axon
Axon
Sensory (afferent) neuron
Motor (efferent) neuron
Cell body
Axon(central process)
Axon(peripheral process)
Sensoryreceptor
Effector(muscle or gland)
Axonterminal
Types of Neuroglial Cellsin the PNS
1) Schwann Cells• Produce myelin found on peripheral myelinated neurons• Speed up neurotransmission
2) Satellite Cells• Support clusters of neuron cell bodies (ganglia)
Types of Neuroglial Cellsin the CNS
1) Microglia• CNS• Phagocytic cell
2) Astrocytes• CNS• Scar tissue• Mop up excess ions, etc.• Induce synapse formation• Connect neurons to blood vessels
3) Oligodendrocytes• CNS• Myelinating cell
4) Ependyma or ependymal• CNS• Ciliated• Line central canal of spinal cord• Line ventricles of brain
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Types of Neuroglial Cells
Microglial cell
Axon
Oligodendrocyte
Astrocyte
Capillary
Neuron
Myelinsheath (cut)
Node ofRanvier
Ependymalcell
Fluid-filled cavityof the brain orspinal cord
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Regeneration of A Nerve Axon
AxonSite of injury Schwann cells
(a)
(b)
(c)
(d)
(e)
Changesover time
Motor neuroncell body
Former connectionreestablished
Schwann cellsproliferate
Schwann cellsdegenerate
Proximal end of injured axon regenerates into tube of sheath cells
Distal portion ofaxon degenerates
Skeletalmuscle fiber
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10.5: The Synapse
Nerve impulses pass from neuron to neuron at synapses, moving from a pre-synaptic neuron to a post-synaptic neuron.
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Synaptic Transmission
• Neurotransmitters are released when impulse reaches synaptic knob
Mitochondrion
Synaptic knob
(a)
Synaptic cleft
Neurotransmitter
Axon
Ca+2
Presynaptic neuron
Direction ofnerve impulse
Synapticvesicles
Cell body or dendriteof postsynaptic neuron
Synapticvesicle
Vesicle releasingneurotransmitter
Axonmembrane
Polarizedmembrane
Depolarizedmembrane
Ca+2Ca+2
10.6: Cell Membrane Potential
A cell membrane is usually electrically charged, or polarized, so that the inside of the membrane is negatively charged with respect to the outside of the membrane (which is then positively charged).
This is as a result of unequal distribution of ions on the inside and the outside of the membrane.
Distribution of Ions
Potassium (K+) ions are the major intracellular positive ions (cations).
Sodium (Na+) ions are the major extracellular positive ions (cations).
This distribution is largely created by the Sodium/Potassium Pump (Na+/K+ pump). This pump actively transports sodium ions out
of the cell and potassium ions into the cell.
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Resting Potential
• Resting Membrane Potential (RMP):
• 70 mV difference from inside to outside of cell• It is a polarized membrane• Inside of cell is negative relative to the outside of the cell• RMP = -70 mV• Due to distribution of ions inside vs. outside• Na+/K+ pump restores
AxonCell body
Low Na+
Axon terminalLow K+
High K+
High Na+
(a)
+
+
––
+
+
––
+
+
–+–
–+–+
–
+–
+–
+–
+–
+–
+–
+–
+–
+ –
–70 mV
(b)
+
+
––
+
+
––
+
+
–+–
–+–+
–
+–
+–
+ –
+–
+–
+–
+–
–70 mV
Low Na+
Low K+ High K+
High Na+
Na+
K+
(c)
Pump
Impermeantanions
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Local Potential Changes
Caused by various stimuli: Temperature changes Light Pressure
Environmental changes affect the membrane potential by opening a gated ion channel
Channels are 1) chemically gated, 2) voltage gated, or 3) mechanically gated
Local Potential Changes
If membrane potential becomes more negative, it has hyperpolarized
If membrane potential becomes less negative, it has depolarized
Graded (or proportional) to intensity of stimulation reaching threshold potential
Reaching threshold potential results in a nerve impulse, starting an action potential
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Local Potential Changes
–62 mV
Na+
Na+
Neurotransmitter
(a)
–55 mV
Na+Na+ Na+ Na+
Na+
Trigger zone
(b)
Chemically-gatedNa+ channel
Presynapticneuron
Voltage-gatedNa+ channel
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Action Potentials
• At rest, the membrane is polarized (RMP = -70)
• Sodium channels open and membrane depolarizes (toward 0)
• Potassium leaves cytoplasm and membrane repolarizes (+30)
• Threshold stimulus reached (-55)
• Brief period of hyperpolarization (-90)
(a)
Region of depolarization
(b)
Region of repolarization
(c)
–70
–0
–70
–0
–70
–0K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+ K+ K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+ K+ K+ K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+
K+
K+
K+ K+
K+ K+
Na+ Na+ Na+
Na+ Na+ Na+
Thresholdstimulus
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+
K+ K+ K+ K+ K+
Na+ Na+ Na+
Na+ Na+ Na+
K+
K+
K+ K+ K+
K+ K+ K+
Na+ channels openK+ channels closed
K+ channels openNa+ channels closed
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Action Potentials
Milliseconds
10
0
+20
+40
2 3 4 5 6 7 8
Mem
bra
ne
po
ten
tial
(m
illi
vo
lts)
Action potential
Hyperpolarization
–40
–20
–60
–80
Restingpotential
Resting potentialreestablished
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Action Potentials
(a)
Direction of nerve impulse
+ +
+ +
+
– – – – – – – – –
– – – – –– – – –
– – – – –– – – –
– – – – – – – – –
– – – – – – – – –
– – – – – – – – –
+ + + + + + + +
+ + + + + + + + +
(b)
+ +
+ +
++ + + + + + + +
++ + + + + + + +
(c)
+ +
+ +
++ + + ++ + + +
++ + + ++ + + +
Region ofaction potential
All-or-None Response
If a neuron responds at all, it responds completely
A nerve impulse is conducted whenever a stimulus of threshold intensity or above is applied to an axon
All impulses carried on an axon are the same strength
Refractory Period
Absolute Refractory Period Time when threshold stimulus does not start
another action potential
Relative Refractory Period Time when stronger threshold stimulus can
start another action potential
Impulse Conduction
10.3 Clinical Application
Factors Affecting Impulse Conduction
10.7: Synaptic Transmission
This is where released neurotransmitters cross the synaptic cleft and react with specific molecules called receptors in the postsynaptic neuron membrane.
Effects of neurotransmitters vary. Some neurotransmitters may open ion
channels and others may close ion channels.
Synaptic Potentials
• EPSP• Excitatory postsynaptic potential• Graded• Depolarizes membrane of postsynaptic neuron• Action potential of postsynaptic neuron becomes more likely
• IPSP• Inhibitory postsynaptic potential• Graded• Hyperpolarizes membrane of postsynaptic neuron• Action potential of postsynaptic neuron becomes less likely
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Summation of EPSPs and IPSPs
• EPSPs and IPSPs are added together in a process called summation
• More EPSPs lead to greater probability of an action potential
Nucleus
Neuroncell body
Presynapticknob
Presynapticaxon
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Neurotransmitters
Neurotransmitters
Neuropeptides
Neurons in the brain or spinal cord synthesize neuropeptides.
These neuropeptides act as neurotransmitters.
Examples include: Enkephalins Beta endorphin Substance P
10.4 Clinical Application
Opiates in the Human Body
10.8: Impulse Processing
Way the nervous system processes nerve impulses and acts upon them Neuronal Pools
Interneurons Work together to perform a common function May excite or inhibit
Convergence Various sensory receptors Can allow for summation of impulses
Divergence Branching axon Stimulation of many neurons ultimately
Neuronal Pools
Groups of interneurons that make synaptic connections with each other
Interneurons work together to perform a common function
Each pool receives input from other neurons
Each pool generates output to other neurons
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Convergence
• Neuron receives input from several neurons
• Incoming impulses represent information from different types of sensory receptors
• Allows nervous system to collect, process, and respond to information
• Makes it possible for a neuron to sum impulses from different sources
1 2
3
(a) 46
45
Divergence
• One neuron sends impulses to several neurons
• Can amplify an impulse
• Impulse from a single neuron in CNS may be amplified to activate enough motor units needed for muscle contraction
(b)
4
5
6
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Important Points in Chapter 10:
10.1: Introduction Describe the general functions of the nervous
system. Identify the two types of cells that comprise nervous
tissue. Identify the two major groups of nervous system
organs.10.2: General Functions of the Nervous System List the functions of sensory receptors. Describe how the nervous system responds to
stimuli.10.3: Description of Cells of the Nervous System Describe the three major parts of a neuron. Define neurofibrils and chromatophilic substance.
Important Points in Chapter 10:
Describe the relationship among myelin, the neurilemma, and the nodes of Ranvier.
Distinguish between the sources of white matter and gray matter.
10.4: Classification of Neurons and Neuroglia Identify structural and functional differences among
neurons. Identify the types of neuroglia in the central nervous
system and their functions. Describe the Schwann cells of the peripheral
nervous system.10.5: The Synapse Define presynaptic and postsynaptic. Explain how information passes from a presynaptic
to a postsynaptic neuron.
Important Points in Chapter 10:
10.6: Cell Membrane Potential Explain how a cell membrane becomes polarized. Define resting potential, local potential, and action
potential. Describe the events leading to the conduction of a
nerve impulse. Compare nerve impulse conduction in myelinated
and unmyelinated neurons.10.7: Synaptic Transmission Identify the changes in membrane potential
associated with excitatory and inhibitory neurotransmitters.
10.8: Impulse Processing Describe the basic ways in which the nervous
system processes information.
The End